1. Field of the Invention
The invention relates to a solid electrolyte capacitor using a conducting polymer as an electrolyte layer.
2. Description of Related Art
Solid electrolyte capacitors have wide applications in such various electronic apparatus as personal computers, mobile electronic apparatus, game machines, and servers. The solid electrolyte capacitors are used mainly for the purpose of smoothing the voltage in power supply circuits and removing current noise.
A typical electronic apparatus includes plural integrated circuits, such as ICs and LSIs. The operation of each IC or LSI causes instantaneous current flow, resulting in a drop of voltage. Thus, in order to operate the electronic apparatus normally by preventing such voltage drop from affecting peripheral circuits and from generating noise, it is necessary to provide, around the ICs and LSIs, capacitors to compensate for the voltage drop or to remove AC noise of high frequencies.
Such capacitors provided to smooth power-supply voltage or to remove noise are also called decoupling capacitors, bypass capacitors, or the like, and are required to have properties of providing high capacitance and having low equivalent series resistances (ESR) in a wide frequency range.
Electrolytic capacitors are a type of capacitor meeting the above requirements. An electrolytic capacitor has an anode body made of a metal such as aluminum, tantalum, or niobium, and includes a dielectric coating formed by transforming the surface of the metal into an insulator through a chemical treatment such as an anodizing process. The electrolytic capacitor uses as a cathode an electrolytic solution or a solid conducting material. Solid electrolyte capacitors are excellent in reliability for a long time because there is no risk of leaking or evaporating of the electrolytic solution.
Some solid electrolyte capacitors include an electrolyte layer made of such inorganic material as manganese dioxide while some others include an electrolyte layer made of such organic material as a conducting polymer or a charge-transfer complex. Organic solid electrolyte capacitors employing a conducting material or charge-transfer complex are often used as decoupling capacitors or bypass capacitors because of their low ESR values.
Since electronic apparatus of recent years, such as computers, have been undergoing rapid progress in size reduction and increased speed, the specs required for the capacitors used in such electronic circuits as mother boards have become much higher than before. Capacitors smaller in size and capable of providing higher capacitance are in demand.
In the meantime, laptop computers and mobile electronic apparatus are strongly expected to consume less power to operate with their batteries for a longer time, and therefore require capacitors having a very small leakage current.
In addition, capacitors need to be highly reliable, and are required to keep their insulating performance and keep functioning as a capacitor without causing any trouble such as short a circuit even if they operate with voltages applied under various temperature conditions.
According to the basic principles of capacitors, a capacitor providing a higher capacitance needs to increase the surface area, decrease the thickness of the dielectric layer, or increase the dielectric constant of the dielectric layer. With the size of the capacitor and the material for the dielectric layer constrained, however, reducing the thickness of the dielectric layer is an important means for increasing the capacitance.
Reducing the thickness of the dielectric layer, however, more likely causes leakage current to flow through the thinner portions or defective portions of the dielectric layer. In addition, the thinner dielectric layer lowers the breakdown voltage of the capacitor under normal or instantaneous application of a voltage. The lower breakdown voltage more likely causes such malfunctions as short circuits.
Patent Document 1 (JP 2-249221A) proposes that either an ion-exchanger polymer compound, such as polystyrene sulfonate and polyvinyl sulfonate, or a polymer compound such as polyvinyl alcohol and polyvinyl chloride is formed on a dielectric oxide coat; and that a conducting polymer compound is formed on the aforementioned polymer compound. JP 2-249221A states that the formation of the polymer compound helps the conducting polymer compound to adhere more strongly to the dielectric oxide coat and allows the conducting polymer compound layer to be formed flatter and smoother.
JP 2-249221A, however, does not disclose any technique enabling the fabrication of a solid electrolyte capacitor that has a smaller leakage current, that is less likely to cause malfunction, and that is highly reliable even when used under higher-temperature and load conditions.
In addition, Patent Document 2 (JP 2001-307958A) proposes a technique including: forming a dielectric oxide coat; then forming a conducting polymer layer as a pre-coat layer; then forming a first conducting polymer layer by electropolymerization and forming a second and a third conducting polymer layer by chemical polymerization.
JP 2001-307958A, however, still fails to disclose any technique enabling the fabrication of a solid electrolyte capacitor that has a smaller leakage current, and that is highly reliable even when used under higher-temperature and load conditions.